Hollow board

ABSTRACT

The present disclosure relates to a hollow board  1  with first and second main surface layers  3, 5 . A plurality of distance elements connecting the first and second main surface layers and maintain a predetermined distance there between. The main surface layers include at least a layer of high-density fiber, HDF, board, and a plurality of distance elements are distributed in the space between the main surface layers, and at least some comprise at least one elongate HDF board strip  15  which is oriented such that its longitudinal edges interconnect the first and second main surface layers  3, 5 . The HDF boards of the surface layers and of the at least some of the distance elements comprise wood particles bonded by a resin including an isocyanate, such as methylene diphenyl di-isocyanate, MDI.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/313,453, filed 26 Dec. 2018, which is a National Stage Application ofPCT/SE2017/050700, filed 26 Jun. 2017, which claims the benefit ofpriority to Swedish Patent Application No. 1650917-6, filed 27 Jun.2016, which applications are incorporated herein by reference. To theextent appropriate, a claim of priority is made to each of the abovedisclosed applications.

TECHNICAL FIELD

The present disclosure relates to a hollow board, comprising first andsecond surface layers and a plurality of distance elements connectingthe first and second surface layers and maintaining a predetermineddistance between the first and second surface layers.

BACKGROUND

Such a board is disclosed for instance in WO-2010/049418-A1, describinga board making up a furniture element with first and second main sides.The board has a supporting structure comprising a number of longitudinalbodies and a hollow distance material placed in between the longitudinalbodies. The longitudinal members may consist of wood or a boardmaterial, and a honeycomb cardboard may constitute the distancematerial. A board material which may be laminated with a foil is adaptedto be folded over and glued on the supporting structure to provide thesurface layers of the first and second sides as well as one or more ofthe board edge surfaces. When the board is finished the supportstructure maintains a predetermined distance between the surface layersof the first and second sides, and the board thereby has a considerablebending stiffness despite being very light.

A general problem in this technical field is how to provide an improvedboard. An improvement may mean a board that is one or more of lighter,stronger, less expensive to produce, having an improved capability ofbeing used in a broad range of working environments, or having a lesserenvironmental impact.

SUMMARY

One object of the present disclosure is thus to provide an improvedhollow board. This object is achieved by means of a hollow board asdefined in claim 1. More specifically, in a board of the initiallymentioned kind, each main surface layer may include at least a layer offiberboard having a density of at least 800 kg/m³ (HDF board), and aplurality of distance elements may be distributed in a space between themain surface layers. At least some of said plurality of distanceelements comprises at least one elongate HDF board strip being orientedsuch that its longitudinal edges interconnect the first and second mainsurface layers. The HDF boards of the surface layers and at least someof said distance elements comprise wood particles bonded by a resinincluding an isocyanate component.

It has been found that by using a resin with an isocyanate component,rather than the typically used formaldehyde based resins, a hollow boardthat is less affected by changing humidity, in particular high humidityand high temperatures, can be achieved. This allows a board to be usedall over the world, while retaining its form and strength. By arrangingthe distance element board strips such that their longitudinal edgesinterconnect the main surface layers, this effect is enhanced. Anyswelling that may still take place in the strip will to a great extentoccur in the direction that the strip material was pressed, and thisdoes not affect its function as a distance element. For instance, it canbe avoided that the swelling of the board strip cracks a surface layeron the side edge surfaces of the hollow board.

The HDF board strip may extend in a plane perpendicular to the plane ofthe first and second main surface layers. This provides a local I-beamwhich entails a hollow board with high bending stiffness about an axisperpendicular to the direction in which the board strip extends. Thehollow board may therefore have a main direction of extension and theHDF board strip an elongate direction being parallel with the maindirection of extension.

The HDF board strip longitudinal edges, that interconnect the mainsurface layers, may be cut edges formed by cutting the strip from a HDFboard. The strip side faces/edges being perpendicular to thelongitudinal edges may have a smoother surfaces than the cutlongitudinal edges, preferably the strip side edges may be press formedside edges. The rougher cut longitudinal edges are well suited for beingglued to the main surface layers.

The HDF board strip may extend along at least 80%, preferably at least90%, of the total length of the board in the main direction ofextension, thus providing a very strong board.

The plurality of distance elements may include at least one stack ofglued together HDF board strips, wherein the stack is oriented inbetween the first and second surface layers such that individual boardstrips in the stack interconnect the first and second surface layers.This provides the same advantages of mainly swelling in a direction thatdoes not affect the hollow board structure as does the single HDF boardstrip, but also a wider piece that may allow the use of connectorelements. Typically, the stack of glued together HDF board strips maycomprise 3-10 individual HDF board strips. This provides a suitablethickness of the stack, to provide extra strength thereof.

A stack may adjoin a side edge of the board. An advantage of thisembodiment is that the stack reinforces the side edge of the board andmakes it more resistant to impacts etc.

Preferably the stack may have a length (L) of less than 20% of the totallength of the board. This reduces the weight of the board, in particularin applications where the stack is needed mainly for local reinforcementof the board.

Preferably the stack may have a width (W) of less than 20% of the totalwidth of the board. This reduces the weight of the board, in particularin applications where the stack is needed mainly for localreinforcement, and/or is needed for side edge reinforcement of theboard.

At least one connector element may be machined in the hollow board atthe location of a block located between the first and second surfacelayers. This block could preferably be a stack of glued together HDFboard strips. The at least one connector element extends at least partlyinto the block, for example the stack of glued together HDF boardstrips. The fact that the connector element is at least partly machinedin the block, such as in a stack of glued together HDF strips, makes theconnector element stronger and allows the hollow board for instance tobe connected to other furniture components in a secure manner.

Preferably the at least one block has a length (L) constituting lessthan 20% of the total length of the hollow board. This provides for ablock that is located locally where the connector element is needed,thereby not adding unnecessary weight to the board.

Preferably the at least one block has a width (W) constituting less than20% of the total width of the hollow board. This too provides for ablock that is located locally where the connector element is needed,thereby not adding unnecessary weight.

The stack of HDF board strips may be glued together using a polyurethanebased reactive hotmelt glue.

Typically, the plurality of distance elements may include both at leastone stack of glued HDF board strips and at least one distance elementcomprising a single HDF board strip, the latter being spaced apart fromsaid at least one stack.

The first and second main surface layers and at least one side edgesurface of the hollow board are preferably made from a single piece ofHDF board. A method and an arrangement for folding a board to form firstand second main surface layers is, for example, described in Swedishpatent application No. SE 1550962-3. This, in addition to providing anefficient manufacturing procedure, verifies that the main surface layersalways come from the same batch of HDF board, and thus have identicalwater content, etc., when being assembled into a hollow board. Thisprovides an improved quality as the main surface layers will behaveidentically once being attached to each other with the distance elementstherebetween.

The isocyanate component of the resin may comprise at least onecomponent selected among methylene diphenyl di-isocyanate (MDI) andpolymethylene polyphenylene isocyanate. These types of isocyanatecomponents have proven very efficient for obtaining a hollow board withgood humidity resistance. Preferably the resin comprises at least 30% ofpolymethylene polyphenylene isocyanate and/or methylene diphenyldi-isocyanate (MDI). This provides for further improved humidityresistance. Still more preferably the resin comprises a 4, 4′-methylenediphenyl di-isocyanate isomer, and/or a polymethylene polyphenyleneisocyanate that has been formed from a 4, 4′-methylene diphenyldi-isocyanate isomer. The 4, 4′-methylene diphenyl di-isocyanate isomerand polymers made therefrom are particularly efficient for formingstrong and humidity resistant hollow boards.

The HDF board may comprise 0.5-15, more preferably 2-10, and mostpreferably 3-7, wt %, excluding any water, of resin containing theisocyanate component. In this context the “wt % of resin” is calculatedexcluding any water, for example excluding any water forming part of theresin formulation and excluding any water used to make an emulsion fromthe resin before mixing it with the wood fibers. The term “excluding anywater” means, hence, that any water added to the resin formulation, forexample to make an emulsion for making it easier to supply the resin tothe wood fiber, is disregarded when calculating the “wt % of resin”. Inother words, the “wt % of resin” refers to the amount of “water free”resin. The above noted resin contents have been found to provide astrong HDF board, without undue cost, that has good resistance to humidenvironments.

Preferably the HDF board has a density of 850-1050 kg/m3. Thesedensities have been found to provide a strong board suitable forproducing hollow boards that are strong and have a low weight.

Preferably the HDF board has a thickness of 0.5-6 mm, more preferably1-3.5 mm. These thicknesses have been found suitable for manufacturing ahollow board that is strong and has a low weight, and is still resistantto humid environments.

More preferably, the HDF board has a thickness of 1.0 to 2.4 mm, evenmore preferred a thickness of 1.5 to 2.2 mm, and still more preferably athickness of about 2.0 mm. Such thicknesses provide for manufacturing ahollow boards that has an even more beneficial relation between highstrength and low weight.

Preferably the HDF board comprises at least 50 wt %, more preferably atleast 80 wt %, of dry wood fibers. While the HDF board may compriseother components, for example scrap plastics, the HDF board becomesstronger, and its behavior more predictable the more wood fibers itcomprises. This is beneficial for the strength and humidity resistanceof the hollow board.

At least one distance element may be glued to the first and second mainsurface layers using a hotmelt glue, preferably a polyurethane basedreactive hotmelt glue.

Preferably all distance elements are made from HDF boards comprisingwood particles bonded by a resin including an isocyanate component. Whenall distance elements of the hollow board are made from this materialthe hollow board obtains predictable properties, and very goodresistance to humid environments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically in perspective a board that can be usedas a furniture component.

FIG. 2 shows in perspective a partial cut-out of a hollow boardaccording to the present disclosure.

FIG. 3 illustrates schematically a HDF board according to the presentdisclosure.

FIGS. 4 a -7 illustrate enlarged details of the board in FIG. 2 .

FIG. 8 shows schematically a cross section through a connection betweena main surface layer and an HDF board strip.

DETAILED DESCRIPTION

The present disclosure relates to a hollow board that can be used as afurniture component. Typically, the board can make up a tabletop, ashelf, a panel in a kitchen cabinet, a door, etc. Many otherapplications exist. FIG. 1 illustrates schematically in perspective sucha hollow board 1. The hollow board comprises a first 3 and a second 5main surface layer, and generally makes up a flat rectangular cuboid,even though deviations from rectangular shapes, e.g. a parallelepipedcould be conceivable. The main surface layers may be quadratic or may asillustrated be elongated having a main direction of extension 7 in whichthe board has its largest dimension.

In the flat, rectangular cuboid, the first and second main surfacelayers may in most cases be interconnected at their four edges by edgesurface layers 9, 11, although it could be conceivable in someapplications to forego from using one or more of the edge surfacelayers, leaving an opening between the main surface layers at thoselocations. In most cases, the main surface layers make up most of theouter surface area of the board.

The board is hollow meaning that a lot of air is enclosed therein, or inprinciple another gaseous medium. This provides a much lighter board ascompared to a solid one. Still, a reasonable bending stiffness can beachieved as the mutual separation of the main surface layers 3, 5provides an increased second moment of area, as compared to if the mainsurface layers were not separated.

In order to provide structural stability, distance elements areprovided, not only at the edges of the board, but spread out in thespace between the main surfaces layers of the board, as will bediscussed. Thus, there is provided a plurality of distance elementsconnecting the first 3 and second 5 main surface layers and maintaininga predetermined distance there between.

In the present disclosure, the main surface layers 3, 5 as well as atleast some distance members comprise a fiberboard having a density of atleast 800 kg/m³, also called high density fiber board or HDF board.

FIG. 3 illustrates schematically a HDF board 13 according to the presentdisclosure. The HDF board 13 is produced by mixing wood fibers/particlessuch as chips or saw dust with a resin at a raised temperature. Themixture is pressed in the illustrated z-direction at a pressure ofseveral MPa while the resin cures. The pressing of the board 13 formsopposite pressed large faces 13 a, 13 b that may be smooth, as a resultof the pressing. The finished product will have very uniform propertiesover its entire surface.

In the present disclosure, the HDF board material may be about 2.0 mmthick, or within the range 1.0 to 2.4 mm, or more preferred within 1.5to 2.2 mm.

Even though the HDF board may be used on its own, it may as well belaminated with other materials. For instance, the HDF board surfacesthat will make up the outer surface of the hollow board may be laminatedwith a decorative foil, such as a polypropylene foil or polyolefin foilor another suitable foil that provide a desired property such as beinghydrophobic. Veneer is another possible option.

“HDF board” as used in the current application means a fiberboardmaterial having a density of at least 800 kg/m³. According to apreferred embodiment the HDF board has a density of 850-1050 kg/m³.

In many cases, conventional HDF boards as well as medium-density fiber,MDF, boards have been produced using a urea formaldehyde compositionresin.

In the present disclosure a resin comprising an isocyanate component hasbeen found to provide useful properties for a hollow board. It has beenfound that a HDF board produced with this resin has a lower hygroscopicexpansion than has a board produced with a urea formaldehydecomposition. This is an important advantage if the product for instanceis produced under conditions with low temperatures and low humidity, andis subsequently used under conditions with high temperatures and highhumidity. In a case where a significant swelling takes place it islikely for instance that a laminated foil on an edge surface cracks, assuch a foil, particularly if made in a plastic material, itself does notexpand substantially due to the increased humidity and temperature.Further, in some cases, the swelling may render the board useless, forinstance if it does not longer fit together with another component thatis not affected in the same way. Still further a HDF board produced witha resin comprising an isocyanate component has also been found to have ahigher Modulus of Elasticity at high relative humidity, compared toprior art HDF boards.

More particularly, the isocyanate component may comprise polymethylenepolyphenylene isocyanate (also called Isocyanic acid,Polymethylenepolyphenylene ester) and/or methylene diphenyldi-isocyanate, MDI. In case the isocyanate component of the resincomprises methylene diphenyl di-isocyanate then it is preferably the4,4′ isomer thereof (4,4′-methylene diphenyl di-isocyanate). Workingexamples involving the above referenced isocyanate components will bedescribed hereinafter.

Details of an example of a structure according to the present disclosurewill now be described with reference to FIG. 2 showing a hollow boardwith one main surface layer partially cut away, and FIGS. 4 a -8 showingenlarged details of FIG. 2 . FIG. 4 a shows enlarged the detail A inFIG. 2 .

A plurality of distance elements 14 may be distributed in a space 16formed between the main surface layers 3, 5. Those distance elements mayinclude, as shown in FIG. 4 a , an elongate HDF board strip 15 producedas mentioned above. This strip 15 has a width corresponding to thedistance between the main surface layers 3, 5, and is oriented standingon its edge such that its longitudinal edges 17 interconnect the firstand second main surface layers 3, 5 such that the space 16 is formedbetween them. The HDF board strip 15 extends in a plane perpendicular tothe planes of the first and second main surface layers, and itslongitudinal edges 17 are glued to the main surface layers 3, 5. Thismay be achieved by e.g. applying a layer of glue to the inner surfacesof the first and second main surface layers 3, 5. A polyurethane basedreactive hotmelt glue, such as RAP IDEX® HL 9554 F, available from H. B.Fuller Europe GmbH, Zurich, CH, may be used to this end.

While it would be possible to use a wider HDF board strip that isinclined with respect to the direction perpendicular to the planes ofthe first and second surface layers, the perpendicular configuration isstructurally preferred. By gluing the HDF board strip such that itextends in a plane perpendicular to the planes of main surface layers, alocal I-beam is formed where the HDF board strip constitutes the beamweb and the main surface layers constitutes the beam flanges. Thisconfiguration provides significant bending stiffness about an axis thatis parallel to the plane of the main surface layers and perpendicular tothe direction in which the HDF board strip extends.

When, as illustrated in FIG. 1 , the hollow board has a longitudinaldirection 7 of extension, the HDF board strip 15 may run straight and inparallel with this direction 7.

This allows the board to take up a greater load, for instance if used asa bookshelf. It may be preferable to let the HDF board strip extendalong the full length of the hollow board, from one short edge 11 to theother, and to glue the ends of the strip to the board making up thoseshort edges. However, this is not necessary to obtain an improvedbending stiffness. For instance, an 80% extension of the length of theboard may be enough for some applications. A plurality of HDF boardstrips 15 may be used spaced apart at even distances over the surface.

Another advantage of arranging an HDF board strip 15 in this way is thatmost of the hygroscopic expansion that may take place, despite thechoice of resin, will take place in what was the z-direction of the HDFboard 13, as illustrated in FIG. 3 , from which the HDF board strip 15was derived. This is due to the board 13 being pressed in this directionduring production. Any hygroscopic expansion of the HDF board strip 15taking place in what was the orthogonal x- and y-directions of the HDFboard 13, see FIG. 3 , from which the HDF board strip 15 was derivedwill most likely be negligible in comparison. This means that the HDFboard strip 15 illustrated in FIG. 4 a can be allowed to expand almostwithout affecting the hollow board at all, because any such expansion ofthe board strip 15, occurring in the z-direction of the original HDFboard 13, will occur in an expansion direction ED being parallel to theplane of the surface layers 3, 5, meaning that the expansion will occurin the hollow space available between adjacent distance elements 14.FIG. 4 b illustrates this preferred arrangement of the HDF board strip15, as seen in cross-section, in which the longitudinal edges 17, thatare to be glued to the main surface layers 3, 5, are cut edges, with adegree of roughness caused by cutting the board strip 15 from a largeboard, such as the board 13 illustrated in FIG. 3 , while the sidefaces/edges 18 of the board strip 15 are surfaces formed already in theHDF board forming process, i.e. the side faces/edges 18 originate fromthe pressed large surfaces 13 a, 13 b of the board 13 illustrated inFIG. 3 , and are as such more smooth than the longitudinal edges 17. Thecutting of the strips 15 from the large board 13 could be made by meansof a saw, a knife arrangement or any other suitable means.

There may also be provided as distance elements stacks 19 comprisingglued HDF board strips 15, which may be glued together using the sameglue as is used to attach the single strips 15 to the main surfacelayers 3, 5. The stack 19 is oriented in between the first and secondsurface layers such that individual board strips in the stackinterconnect the first and second surface layers, i.e. the individuallayers in the stack are oriented in the same way as the single strips.Such stacks 19 form laths that provide additional strength where needed.As shown in FIG. 2 this may be useful at side edges of the hollow boardand at other locations where the hollow board need be connected to othercomponents as will be discussed.

FIG. 5 shows enlarged the detail B in FIG. 2 . A block in the form of ashort stack 21 of HDF board strips may as shown be arranged to adjoin anedge 11 of the board. The short stack 21 may as illustrated be glued toanother stack 19, and is used to provide basis for a connector element,in the illustrated case a hole 23 for a screw or the like. The shortstack 21 is therefore only needed in the vicinity of the location of theconnector 23. In an alternative embodiment, a connector 23 is located atan arbitrary position in a main surface layer 3, 5, for example tosupport a shelf of a book shelf or to support a handle of a door. If so,the short stack is only needed at that location, and does not need toadjoin an edge. Preferably the short stack 21 may have a length Lextending less than 20% of the total length of the board in theextension direction of the stack strips, or in any case the short stack21 extending less than half of the total length of the board. Such ashort stack 21 therefore adds only little extra weight and still mayprovide a useful additional function. In general, the stack 19, 21 widthW, as seen perpendicular to the extension direction of the stack stripsof the stack 19, 21, may preferably extend less than 20% of the totalwidth of the board to save weight.

As alternative to, or in combination with, a block having the form of ashort stack 21 made from glued together HDF board strips, a block mayalso be made from other materials. For example, a block may be made fromplastic or metal, and may be located between the surface layers 3, 5 forthe purpose of providing various functionalities, for example as will bedescribed hereinafter.

Connector elements may also be added in the main surface layers. FIG. 6shows enlarged the detail C in FIG. 2 . Here connector elements in theform of drilled holes 25 are provided in the main surface layer 3 at alocation where a stack 19 of glued HDF board strips is provided beneaththe main surface layer 3. Such holes 25 may for instance be used to fitplugs or screws for connecting the hollow board to another element.Another example is illustrated in FIG. 7 showing enlarged the detail Din FIG. 2 . Here a connector element in the form of a groove 27 isprovided in the main surface layer 3 at a location where a stack 19 ofglued HDF board strips is provided beneath the main surface layer 3. Insuch a groove 25, a HDF boar may be slid in, for instance.

FIG. 8 shows a cross section through a connection between an HDF boardstrip 15 and a main surface layer 3. As illustrated, the main surfacelayer 3 comprises a main surface layer HDF board 29 with a main surfacelayer foil 31 laminated thereon to present the outer surface 35 of themain surface layer 3. A glue layer 33 is provided on the inner surface37 of the main surface layer 3, and adheres to the HDF board strip 15,at least to the strip edge 17 thereof. The glue layer 33 is thus appliedjust before the board strips 15 are assembled with the main surfacelayer 3.

Example 1

An HDF board was produced in the following manner: Wood based fibershaving an average length of about 5-20 mm and an average diameter ofabout 0.05-0.3 mm were formed by pressing, and refining wood chips andthen drying the resulting fibers according to normal HDF productionprocedures. The wood fibers were mixed with a resin called I-BOND® MDFEM 4330, which is available from Huntsman Holland BV, Botlek-Rotterdam,Netherlands. This resin comprises, as a major active ingredient,Polymethylene polyphenylene isocyanate (CAS: 9016-87-9) (sometimes alsoreferred also to as Isocyanic acid, Polymethylenepolyphenylene ester).The resulting wood fiber mixture comprised about 5.5 wt % moisture, 4.5wt % resin, excluding any water, and the remaining part, i.e. 90 wt %,was dry wood fibers. This wood fiber mixture was used to form a fibermat of 12 mm thickness, which was introduced into an HDF board press.The fiber mat was, during a period of about 3 seconds, exposed to a highpressure treatment which involved exposing the fiber mat to a pressurethat increased up to 4 N/mm² and that was then reduced again. Thetemperature was about 200° C. during this initial high pressuretreatment. After the high pressure treatment the pressure was about 0.4N/mm² and this pressure was maintained for about 5 seconds, combinedwith a temperature of about 160° C. Then pressure was increased to about1.5 N/mm2, temperature was maintained at about 160° C., and this finalpressing was performed during about 9 seconds. In total the pressingoccurred during 3+5+9=17 seconds, and the resulting HDF board had athickness of about 2 mm, a density of 940 kg/m3 and a moisture contentof about 4 wt % after pressing.

Example 2

Wood fibers were prepared in a similar manner as in Example 1. The woodfibers were mixed with a resin called I-BOND® MDF PM 4390 available fromHuntsman Holland BV, Botlek-Rotterdam, Netherlands. This resincomprises, as major active ingredients, Polymethylene polyphenyleneisocyanate (CAS: 9016-87-9), (sometimes also referred also to asIsocyanic acid, Polymethylenepolyphenylene ester), and4,4′-Methylenediphenyl diisocyanate (CAS: 101-68-8). The resulting woodfiber mixture comprised about 5.5 wt % moisture, 4.5 wt % resin,excluding any water, and the remaining part, i.e. 90 wt %, dry woodfibers. This wood fiber mixture was used to form a fiber mat of 12 mmthickness, which was introduced into an HDF board press. The fiber matwas pressed following the same sequence as described above forExample 1. The resulting HDF board had a thickness of about 2 mm, adensity of 940 kg/m3 and a moisture content of about 4 wt % afterpressing.

Comparative Example

The comparative example HDF board was prepared in a similar manner asExamples 1 and 2, with the difference that the resin used was acommercially available Melamine Urea Formaldehyde (MUF). A wood fibermixture comprised about 8 wt % moisture, 10 wt % resin and the remainingpart, i.e. 82 wt %, dry wood fibers. This wood fiber mixture was used toform a fiber mat of 12 mm thickness, which was introduced into an HDFboard press in which an HDF board was formed with a thickness of about 2mm, a density of 940 kg/m3 and a moisture content of about 4 wt % afterpressing.

Results:

The HDF boards produced according to the above techniques were exposedto different degrees of humidity and were then tested for modulus ofelasticity (MoE) according to EN310. Table 1 below presents the testresults. The column to the right in table 1 presents for each examplethe Modulus of Elasticity at 90% relative humidity divided by theModulus of Elasticity at 60% relative humidity. The closer to 1 thatthis value is the less is the board affected by humidity:

TABLE 1 Modulus of Elasticity (MoE), (N/mm²), at different relativeHumidity Relative Humidity MoE at 90%/ 60% 70% 80% 90% MoE at 60%Example 1 5700 5200 4700 3200 0.56 Example 2 4800 4500 4000 3200 0.67Comparative 5200 4300 2900 2200 0.42 Example

As can be seen from table 1 the HDF board of the Comparative Example isheavily affected at high relative humidity. The HDF-boards of Examples 1and 2 on the other hand resists humidity much better and at 90% relativehumidity still have substantially more than 50% of their Modulus ofElasticity at 60% relative humidity.

The present disclosure is not restricted to the above examples, and maybe varied and altered in different ways within the scope of the appendedclaims. For example, it has been described hereinabove that all distanceelements 14, 19 are made from HDF boards comprising wood particlesbonded by a resin including an isocyanate component. While this is apreferred embodiment, it is in some applications possible to use somedistance elements made from another material. For example, some of thedistance elements could be made of MDF, having a lower density than HDF.MDF would have a density of less than 800 kg/m³, more typically adensity of 600-780 kg/m3. In such an embodiment at least the distanceelements located close to the edges of the hollow board are preferablymade from HDF, while the central distance elements may be made from MDF,or another material, since the hollow board is somewhat less sensitiveto humidity induced swelling at its central portions.

The invention claimed is:
 1. Hollow board comprising first and secondmain surface layers and a plurality of distance elements connecting thefirst and second main surface layers and maintaining a predetermineddistance there between, wherein (i) each main surface layer including atleast a layer of board having a density of at least 800 kg/m³, and (ii)a plurality of distance elements distributed in a space between the mainsurface layers, and at least some of said plurality of distance elementscomprising at least one elongate board strip, having a density of atleast 800 kg/m³ and being oriented such that its longitudinal edgesinterconnect the first and second main surface layers, wherein theplurality of distance elements includes at least one stack of gluedtogether board strips having a density of at least 800 kg/m³, whereinthe stack is oriented in between the first and second surface layerssuch that individual board strips in the stack interconnect the firstand second surface layers.
 2. Hollow board according to claim 1, whereinthe board of at least one of the main surface layers comprises woodparticles.
 3. Hollow board according to claim 2, wherein the woodparticles of the board of at least one of the main surface layers arebonded by a resin.
 4. Hollow board according to claim 1, wherein atleast one of the board strips of the plurality of distance elementscomprises wood particles.
 5. Hollow board according to claim 4, whereinthe wood particles of the board strips of the plurality of distanceelements are bonded by a resin.
 6. Hollow board according to claim 1,wherein said at least one board strip extends in a plane perpendicularto the plane of the first and second main surface layers.
 7. Hollowboard according to claim 1, wherein the hollow board has a maindirection of extension and the board strip has an elongate directionbeing parallel with the main direction of extension.
 8. Hollow boardaccording to claim 1, wherein the board strip longitudinal edges thatinterconnect the main surface layers are cut edges formed by cutting thestrip from a board, and wherein the strip side edges being perpendicularto the longitudinal edges have a more smooth surface than the cutlongitudinal edges, preferably the strip side edges being press formedside edges.
 9. Hollow board according to claim 7, wherein the boardstrip extends along at least 80% of the length of the board in the maindirection of extension.
 10. Hollow board according to claim 1, whereinthe at least one stack of glued together board strips comprises 3-10individual board strips.
 11. Hollow board according to claim 1, whereinat least one stack adjoins a side edge of the hollow board.
 12. Hollowboard according to claim 1, wherein at least one connector element ismachined in the hollow board at the location of a block placed betweenthe surface layers, the at least one connector element extending atleast partly into the block.
 13. Hollow board according to claim 12,wherein the at least one block has a length (L) constituting less than20% of the total length of the hollow board.
 14. Hollow board accordingto claim 1, wherein the stack of board strips are glued together using apolyurethane based reactive hotmelt glue.
 15. Hollow board according toclaim 1, wherein the plurality of distance elements includes both atleast one stack of glued together board strips and at least one distanceelement comprising a single board strip having a density of at least 800kg/m³, the latter being spaced apart from said at least one stack. 16.Hollow board according to claim 1, wherein the first and second mainsurface layers and at least one side edge surface of the hollow boardare made from a single piece of board.
 17. Hollow board according toclaim 1, wherein at least one distance element is glued to the first andsecond main surface layers using a hotmelt glue.
 18. Hollow boardaccording to claim 11, wherein the board strip extends along at least90% of the length of the board in the main direction of extension. 19.Hollow board according to claim 10, wherein the at least one stack has awidth (W) constituting less than 20% of the total width of the hollowboard.
 20. Hollow board according to claim 11, wherein the at least onestack adjoining the side edge has a width (W) constituting less than 20%of the total width of the hollow board perpendicular to that edge. 21.Hollow board according to claim 12, wherein the block is a stack ofglued together board strips.
 22. Hollow board according to claim 17,wherein the at least one block having a width (W) constituting less than20% of the total width of the hollow board.
 23. Hollow board accordingto claim 1, wherein the board having a density of 850-1050 kg/m3, athickness of 0.5-6 mm, and comprises at least 50 wt % of dry woodfibers.
 24. Hollow board according to claim 23, wherein the board havinga thickness of 1-3.5 mm, and comprises at least 80 wt % of dry woodfibers.
 25. Hollow board according to claim 21, wherein at least onedistance element is glued to the first and second main surface layersusing a polyurethane based reactive hotmelt glue.